The present invention relates to ultrasonic acoustic combination receivers and transmitters, such as may be used in quantitative ultrasonic imaging and measurements, and in particular to an improved thin film receiver/transmitter.
Quantitative ultrasonic imaging is used, for example, in bone densitometry where ultrasound is transmitted through in vivo bone, most typically the os calcis of the heel, in order to measure trabecular bone. Common measurements made by such densitometers include the speed of sound (SOS) and broadband ultrasonic attenuation (BUA) in the bone. Images of the bone based on these or other measurements may also be provided by the densitometer. Densitometers of this type are described in U.S. Pat. Nos. 5,840,029 and 6,517,487, assigned to the assignee of the present invention, and hereby incorporated by reference.
Ceramic transducers are commonly used as the transmitting ultrasonic transducer in such densitometers because of their high output signals. In this application, the mechanical resonance of the ceramic transducer is adjusted to be near the principal frequency being transmitted. Operation in this “resonant” mode increases the output of the transducer, but can make manufacturing of the transducer difficult because of the high sensitivity of the transducers resonant frequency to variations in the dimensions of the many subcomponents of the transducer. Slight differences in resonant frequencies of the transducers on different machines complicate the effort to provide highly repeatable measurements that are machine independent. Significant differences in transmission frequencies can affect quantitative measurements such as assessments of bone density.
Thin film polymer piezoelectric materials such as polyvinylidene fluoride (PVDF) may also be used as a receiving ultrasonic transducer as described in U.S. Pat. No. 6,305,060 issued Oct. 23, 2001, and U.S. Pat. No. 6,012,779 issued Jan. 11, 2000 assigned to the assignee of the present invention and hereby incorporated by reference. Application of PVDF to transmitting ultrasonic transducers has been limited because of low output levels.